added some efficiency changes via numexpr
Merge branch 'master' into vel
Merge conflicts resolved.  Merged in Luke's changes to implement magnetic_declination from the C WMM model instead of geomag.
Added met_functions and cleaned up some others
added to met_functions
added test_met_functions
fixed function names
Fixes met & vel unit tests
Update data/README.txt
checked of vel test data in TODO list
Merge branch 'master' into spworking
Merge pull request #62 from s-pearce/spworking

METBK functions and clean-ups of vel_functions, generic_functions and readme.